The signal path between two telephones, involving a call other than a local one, requires amplification using a four-wire circuit. The cost and cabling required discourages extending a four-wire circuit to a subscriber's premises from the local exchange. For this reason, the four-wire trunk circuits are coupled to two-wire local circuits, using a device called a hybrid.
Hybrid echo, the primary source of echo generated from the public-switched telephone network (PSTN) is created as voice signals are transmitted across the network via the hybrid connection at the two-wire/four-wire PSTN conversion points.
Unfortunately, the hybrid is by nature a leaky device. As voice signals pass from the four-wire to the two-wire portion of the network, the energy in the four-wire section is reflected back, creating an echo of the speech signal. Provided that the total round-trip delay occurs within just a few milliseconds, the echo results in a user perception that the call is ‘live’ by adding sidetone, thereby making a positive contribution to the quality of the call.
In cases where the total network delay exceeds 36 ms, however, the positive benefits disappear, and intrusive echo results. The actual amount of signal that is reflected back depends on how well the balance circuit of the hybrid matches the two-wire line. In the vast majority of cases, the match is poor, resulting in a considerable level of signal being reflected back.
The effective removal of hybrid echo is one key to maintaining and improving perceived voice quality on a call. This has led to intensive research into the area of echo cancellation, with the aim of providing solutions that can reduce echo from hybrids. By employing the results of this research, the overall speech quality is improved significantly.
It is known in the art to employ adaptive filtering to address hybrid echo cancellation. In an adaptive filter, the filter coefficients are based, in part, on feedback of filter output. Normalized Least Mean Square (NLMS) adaptive filtering is one method, popular in echo cancellation, to address reflections in the telephony system.
In such echo cancellers, the coefficients of an adaptive filter converge to a certain echo path. Under ideal conditions, a generally acceptable convergence time requires that the echo canceller achieve 27 dB of ERLE (Echo Return Loss Enhancement) in 0.5 sec. Once the coefficients are converged, the echo is canceled from the input signal. When the echo path changes (i.e. call transfer, conferencing), the echo canceller has to quickly re-converge to the new echo path or else the echo will be perceived by the user. Detecting line changes is a difficult problem since the echo resulting from a new hybrid in a changed echo path and the echo generated by the old hybrid from the converged adaptive filter can easily be confused as a double talk signal.
Prior art solutions to this problem may be found in U.S. Pat. No. 6,035,034 (Trump, Tonu): Double talk and Echo Path Change Detection in a Telephony System, and U.S. Pat. No. 6,226,380 (Heping, Ding): Method of Distinguishing Between Echo Path Change and Double Talk Conditions in an Echo Canceller.